The present invention relates to a discharging apparatus for a combination battery. The discharging apparatus according to this invention can be, for example, employed in a hybrid-type electric vehicle or other electric vehicle which usually mounts a combination battery consisting of a plurality of, for example, lithium-ion batteries being serially connected.
Recently, from the view point of the protection of the global environment, the lithium-ion batteries are evaluated as prospective batteries which have the capability of producing a driving power for an electric vehicle (EV) or a hybrid-type electric vehicle (HEV).
However, this battery is delicate and weak against overcharge and overdischarge. If this battery is not used within a predetermined voltage range, the constituent materials will be disengaged and accordingly a remarkable reduction in the capacitance or abnormal heat generation will be caused.
Accordingly, in the usage of the lithium-ion batteries, the upper-limit voltage and the lower-limit voltage must be strictly regulated. For example, a constant voltage charge control is performed within the regulated range. A protection circuit for preventing the voltage from deviating outside this voltage range is used.
However, a driving motor equipped in an electric vehicle, such as HEV or EV, requires approximately 300 V. A combination battery installed on an electric vehicle consists of numerous cells being serially connected. For example, the combination battery requires 150 cells in the case of lead batteries (approximately 2V/cell), 250 cells in the case of nickel-metal hydride (1.2V/cell), and approximately 80 cells in case of lithium-ion batteries (3.6V/cell).
According to this kind of combination battery, respective secondary batteries (hereinafter, referred to as “cells” or “unit cells”) are different from each other in the residual capacitance (SOC: state of charge). The difference of secondary batteries in the SOC will cause difference in respective cell voltages (i.e., in respective unit cell voltages).
Namely, according to the combination battery, respective unit cells are different in the value of SOC due to individual difference in the capacitance as well as in the discharge characteristics, and accordingly respective cell voltages are different from each other. According to the charge/discharge control based on a terminal voltage of the combination battery (i.e., combination battery voltage), the control is performed based on an average voltage of respective unit cells. A cell voltage having a cell voltage higher than the average voltage has the tendency of overcharge, while a cell having a cell voltage lower than the average voltage has the tendency of overdischarge.
However, the overcharge durability and the overdischarge durability of the lithium-ion batteries are inferior to those of other kinds of secondary batteries. Furthermore, the organic electrolytic solution of the lithium-ion batteries is not water soluble and hence there is no confined reaction and the uniform charging is unfeasible. The difference among respective cell capacitances will so increase that the combination battery cannot be practically used.
To solve this problem, the Japanese Patent Application Laid-open No. 61-206179 (1986) proposes providing a Zener diode being connected in parallel with each cell to discharge an excessive capacitance exceeding the reverse breakdown voltage via the Zener diode. According to this method, the circuit arrangement is simple. However, it is necessary to add the reverse breakdown voltage to the upper-limit voltage of the battery. Each cell requires a large-scale Zener diode serving as a bypass element for the main current flowing across the combination battery. The cost will so increase that this arrangement cannot be practically used.
To solve the above-described problem, other prior art documents, such as the Japanese Patent Application Laid-open No. 7-336905(1995), the Japanese Patent Application Laid-open No. 2000-92733, and the Japanese Patent Application Laid-open No. 11-150877(1999), disclose a discharging circuit consisting of a discharge resistor and a discharge switch which is connected in parallel with each cell. According to this kind of cell discharge system, the discharging circuit of any cell having a higher terminal voltage is selectively discharged (or the charge current is bypassed) to reduce the voltage difference among respective cells.
However, according to this kind of discharge system, there is a problem that the cell is subjected to overcharge or overdischarge in case of ON failure of the discharge switch (i.e., the failure forcing the discharge switch to always turn on) or OFF failure of the discharge switch (i.e., the failure forcing the discharge switch to always turn off).
The above-described problem also arises when electric double layer capacitors are serially connected. Accordingly, the “cell” described in the following description includes an electric double layer capacitor.